1. Field of the Invention
The present invention relates to an information signal recording apparatus for recording a digital information signal, and particularly, relates to an information signal recording apparatus capable of performing an intermittent recording by running a magnetic tape at a lower running speed than at a normal recording.
2. Description of the Related Art
Presently, there is the so-called "helical scanning type digital VTR", wherein a magnetic tape is wound around a rotary drum, and video and sound digital signals are recorded on the magnetic tape by rotating magnetic heads (referred to as rotary heads hereinafter) mounted on the rotary drum, resulting in a predetermined track pattern.
FIG. 1 is a plan view of a rotary drum for explaining attachment positions of rotary heads provided thereon in a helical scan type recording apparatus;
FIG. 2 is an explanatory view of a track pattern formed on the magnetic tape in a normal recording mode by employing the helical scan type recording apparatus shown in FIG. 1;
FIG. 3 is an explanatory view of an intermittent recording by employing the helical scan type recording apparatus;
FIG. 4 is a circuit diagram employing rotary transformers in the helical scan type recording apparatus;
FIG. 5 is a chart of pulses generated responsive to revolution of the rotary heads which pulses are used in the intermittent recording, and
FIGS. 6(A) and 6(B) are charts for explaining a discharge signal generated in the intermittent recording;
In a normal recording mode (referred to as normal recording) of the helical scanning type digital VTR, the magnetic tape is transferred at a constant speed along the rotary drum while the rotary drum is rotated at a constant speed.
As shown in FIG. 1, in the normal recording, rotary heads a1, b1 having different azimuth angles to each other are mounted on the rotary drum 10 to face each other at 180.degree.. As shown in FIG. 2 tracks A, B are alternately formed on the magnetic tape T responsive to the rotation of the rotary heads a1, b1.
Here, for instance, in contrast to a case that a digital information signal transferred at a certain data rate is recorded in the normal recording mode, when a digital information data transferred at a lower data rate than that in the normal recording mode is recorded, to employ the so-called "intermittent recording" (the time-lapse recording) is effective to increase the capacity of recording.
For instance, when the intermittent recording is performed in such a manner that the tape speed is made to be 1/3 as low as that in the normal recording and one track is formed at every 1.5 revolutions (the number of revolution of the rotary drum 10 is the same speed as that of the normal recording mode), it is possible to form almost the same track pattern as that shown in FIG. 2 by alternately applying a recording signal to each of the rotary heads a1, b1 at one time per 1.5 revolutions of the rotary drum 10.
At the time, the recording time for a given magnetic tape will be 3 times as much as that in the normal recording mode because the tape speed thereof is 1/3 as low as that in the normal one.
When the tape speed is made to be 1/3 as low as that in the normal recording without the change of the number of revolutions of the rotary drum 10, actually, the rotary heads a1, b1 scan on the magnetic tape T, forming tracks of which parts are superimposed to each other as shown in FIG. 3. Specifically, the rotary heads a1, b1 form tracks A1, B1 during one revolution of the rotary drum 10, and tracks A2, B2 during next one revolution and successively tracks A3, B3, A4, B4, A5, B5, A6, B6, . . . (up to B3 are shown).
Accordingly, in order to perform the so-called "intermittent recording", it is possible to obtain such a track pattern as similar to that formed in the normal recording without an interference of the recorded tracks caused by the superimposition of the recording signal by supplying the recording signal to both the rotary heads a1, b1 in such a timing that the rotary heads a1, b1 scan on the tracks A1, B2, A4, B5 . . . which are less superimposed to each other shown in FIG. 3.
In order to perform the intermittent recording by using the rotary heads a1, b1 having a circuit including rotary transformers Ra, Rb as shown in FIG. 4, it is possible to perform the intermittent recording by supplying the recording signal to the rotary heads a1, b1 in such a timing as shown in FIG. 5 in accordance with the revolution of the rotary drum 10.
As shown in FIG. 5, a pulse (DFF: drum flip-flop) of [H] (high) and [L] (low) levels is generated by control pulse generating means (not shown) responsive to the revolution of the rotary heads a1, b1.
For instance, when the pulse (DFF) is [L] level, the rotary head a1 scans on the magnetic tape T, and when the pulse (DFF) is [H] level, the rotary head b1 scans on the magnetic tape T. In addition, when the rotary heads a1, b1 scan on the hatched tracks A1, B2, A4, B5 . . . , the recording signal is applied to both the rotary heads a1, b1, resulting in the track pattern almost the same as that shown in FIG. 2.
However, upon performing the intermittent recording with respect to the tracks A1, B2, A4, B5 . . . , the recording signal is intermittently applied to both the rotary heads a1, b1. Thus, when the rotary heads a1, b1 scan on the tracks B1, A3, B4, A6 . . . (inoperative intervals) thereafter, there is a problem that a discharge signal (current) Sd caused by a differential response of the recording signal occurs in each of the rotary heads a1, b2.
As shown in FIG. 6(B), for instance, when the rotary head a1 is forming the track A1 by scanning on the magnetic tape T, the recording signal is applied to the rotary head b1 as well as the rotary heads a1. Thus, when the rotary head b1 successively scans on the track B1, the discharge signal Sd flows in the rotary head b1 due to the differential response of the recording signal mentioned above.
Specifically, when the recording signal is made of a voltage signal responsive to a binary code of [1] or [0] of a digital information signal, the voltage signal responsive to the code of [1] or [0] is always applied to the rotary heads a1, b1.
When the recording signal to the rotary heads a1, b1, is stopped the state just prior to the stop of the recording signal for recording the track A1 as shown in FIG. 6(B) is maintained for a while in the rotary heads a1, b1. Here, it is difficult for the rotary heads a1, b1 each having the rotary transformer Ra or Rb to transmit the signal of low frequency. Thus, the decaying discharge signal Sd caused by the recording signal just prior to the stop of the recording signal flows through the rotary head b1 while the rotary head b1 scans on the beginning of the track B1.
In other words, upon scanning on the tracks B1, A3, B4, A6 . . . , when the discharge signal Sd is generated in the rotary heads a1, b1, the discharge signal interferes with the tracks A1, B2, A4, B5 . . . where the recording signal is recorded, resulting in a degradation in the S/N of the reproduction signals obtained from the tracks A1, B2, A4, B5 . . .
In order to prevent the discharge signal Sd from generating, it is conceivable to provide a switch to each of the rotary heads a1, b1 for supplying the recording signal. Thereby, the recording signal to the rotary heads a1, b1 is turned on or off responsive to the intermittent operation of the recording signal. However, there arises a problem that such method requires a rather complicated control to switch the recording signal to the plural rotary heads responsive to the revolution of the rotary drum 10.
Further, the more the number of the rotary heads is increased, the more the number of the switches is required, resulting in a complicated circuit and control.